1. Field of the Invention
The present invention relates to an optical head used for an optical memory apparatus for recording/reproducing information on/from an optical disk by light beams thereon and, more particularly, to a rotary-type optical head which traces tracks while rotating by oneself above an optical disk.
2. Description of the Related Art
Optical memory apparatuses for recording/reproducing information by using optical disks have recently been widely used. In such optical memory apparatuses, information signals are recorded/reproduced by tracing spiral grooves formed in an optical disk with a laser beam emitted from an optical disk. This laser beam is focused on a recording layer by an objective lens to have a small beam spot having a diameter of about 1 .mu.m. If the optical disk is deflected, the focusing position of the beam spot is deviated from a proper position on the information recording medium. This is called a focusing error. In addition, if the center of a spiral groove is shifted from the rotation center of the optical disk (called "radial runout"), the groove and the beam spot are shifted from each other upon rotation of the optical disk. This is called a tracking error. In order to control these errors, the optical head includes an error detection system and a control mechanism for controlling the position of the beam spot on the basis of servo error signals (i.e., a focusing error signal, a tracking error signal, and the like) generated by detection outputs from the error detection system. If the beam spot is accurately controlled on the recording layer, information signals can be recorded/reproduced with high reliability.
Error detection is generally performed in the following manner. Light reflected by an optical disk is detected by using a photodetector having several divided sensor segments. Output signals corresponding to the respective sensor segments are then subjected to arithmetic processing such as addition and subtraction, thus detecting an error. For example, as a method of focusing error detection, a detection method shown in FIGS. 15(a) and 15(b) is known. Referring to FIG. 15(a), light reflected by an optical disk is converged by a convergent lens 81. If the optical disk is shifted from the position of a beam spot focused by an objective lens, the reflected light from the optical disk is incident, as convergent or divergent light, on the convergent lens 81. For this reason, the converging position of the light beam transmitted through the convergent lens 81 is changed. This change in focusing position of a light beam is detected by arranging photodetectors 82 and 83, each having three divided sensor segments 29a to 29c shown in FIG. 15(b) at positions separated from the focal point of the convergent lens 81 by the same distance, thus detecting a change in size of the light beam. Tracking control of a conventional optical head is performed by controlling the current amount of a tracking coil 84 to move the objective lens in a direction perpendicular to the tracks. With this operation, a light beam on a photodetector surface moves in accordance with an eccentricity amount. The three divided sensor segments of the photodetector are sequentially arranged in the track direction so as to minimize a focusing error detection offset caused upon movement of the light beam. In this arrangement, an information signal in reproduction is influenced by non-detecting regions (to be referred to as split regions hereinafter) between the three divided sensor segments 29a to 29c, resulting in a deterioration in frequency characteristics of the retrieving information signal.
In addition, as a method of detecting a tracking error, a method shown in FIG. 7 is known. In this method, a photodetector having two divided sensor segments in a direction perpendicular to the tracks is arranged to divide a light beam. A tracking error is detected by subtracting output signals from the two sensor segments from each other. This detection method is a typical tracking error detection method called "push-pull detection method". A region where "diffracted light" and "0th-order light" from a groove in an optical disk overlap (indicated by hatching) is a region where a light amount change caused by a track shift is large.
FIG. 17 is a perspective view showing an outer appearance of an optical head which is driven to be freely rotated about a shaft so as to selectively trace tracks on an optical disk.
According to a rotary-type optical head, upon rotation of the optical head, the projection of a track on an optical disk is rotated on a photodetector. When the projection of the track is rotated, a portion where "diffracted light" and "0th-order light" overlap is also rotated. As a result, a tracking error signal changes. However, according to a rotary-type optical head based on another scheme in which a detection optical system for guiding reflected light from an optical disk to a photodetector is arranged in a fixed optical system which is not rotated with the optical head, since the projection of a track is only slightly rotated on the photodetector, tracking error detection is not much influenced by the rotation of the projection. In contrast to this, in the rotary-type optical head having a compact arrangement in which the overall optical system including the detection optical system is integrated in a rotation drum, since the projection of a track is excessively rotated on the photodetector, tracking error detection is greatly influenced by the rotation. As a result, an accurate tracking error signal cannot be obtained
As described above, it is especially difficult to reproduce accurate information from pits at an inner zone of an optical disk designed to be rotated at a constant angular velocity, because the size of each pin at the inner zone is smaller than that at an outer zone. In addition, as the rotation amount of the projection of a track on the photodetector is increased upon rotation of the rotation drum, an accurate tracking error signal cannot be obtained.